It is well known that various types of device failures are attributable to the presence of moisture. Thus, for example, in sealed packages containing microelectronic devices of the integrated-circuit type, a water vapor concentration as low as several thousand parts per million can cause device failures. Accordingly, efforts have been directed at devising techniques for measuring the moisture level within such a package. By determining this level during the device fabrication process, it is possible initially to identify and reject failure-prone sealed packages. Moreover, by subsequently monitoring the moisture level within packages actually connected in working equipment, it is feasible to identify and replace those packages whose moisture content later rises above acceptable limits due to leakage or to outgassing of materials within the package.
Heretofore, various types of in-situ moisture sensors for inclusion in sealed packages have been suggested. One such known sensor comprises a pair of interdigitated electrodes contained within the package. During a moisture measuring cycle, the entire package is placed in a test chamber whose ambient temperature can be controlled. As the package is cooled, the direct-current conductivity between the electrodes of the sensor is measured. When the dew point of the atmosphere within the package is reached, moisture condenses on the electrodes. At that point, an increase in conductivity between the electrodes is detected. From such a determination of the dew point temperature within the package, the water vapor concentration therein may be directly determined. A dew point moisture sensor of this type is described in "A New Moisture Sensor for In-Situ Monitoring of Sealed Packages", Solid State Technology, February 1978, pages 35-39, 53.
Although the above-described dew point moisture sensor for sealed packages appeared in concept to be attractive, it was found in practice to be insufficiently sensitive and accurate for performing critical low-moisture measurements. Accordingly, applicants and other workers in the field continued to try to devise moisture measuring techniques which would exhibit improved sensitivity and accuracy.